It is known for a worker required to stand and move along a structure located spacedly over ground to use a fall arrest system to avoid falling to the ground. The structure may be, for example, a space frame truss located high over ground in a construction site, theatre, factory, storage facility, outdoor bridge or the like; a scaffolding erected about a structure being constructed or repaired; or an existing structure such as a building or bridge along which the worker is required to move.
Known fall arrest systems comprise a lifeline running along the structure on which the worker will move. The lifeline is an elongated support element that can be flexible such as a cable or wire, or rigid such as a rail. The fall arrest systems also comprise a harness that the worker equips, linked to a carabiner by means of a lanyard. The carabiner consists of a closed loop rigid rod with one segment thereof being a spring catch, to allow easy attachment to the lifeline. However, direct attachment of the carabiner to the lifeline means that if the worker falls off the structure on which he is standing, although he will be prevented from falling to the ground beyond the length of his lanyard due to attachment to the lifeline, he can still be undesirably dragged under his own weight to one side or the other along the lifeline if the latter is inclined or sags. Such a situation may result in the worker being injured even if his fall to the ground is prevented.
The prior art discloses some carriages, or trolleys, that may be used as part of the fall arrest systems. The purpose of these trolleys is to slide or roll along the lifeline when the worker desires to move along it, but to lock or brake against the lifeline if the worker falls, to arrest this fall. The worker attaches himself with his carabiner to the trolley. They prior art trolleys known to the inventors fall into two categories: those for use along vertical cables and those for use along inclined cables. Inclined cables, as the name suggests, are those that are neither horizontal nor vertical.
Prior art trolleys for use along vertical cables or rails are used by workers that move vertically along a structure. These trolleys come in different designs and shapes, but have a common feature: it is upon the weight of the worker falling downwards, parallel to the vertical cable, that the trolley fall arrest mechanism will be activated. Many such trolleys include a cam pivotable on a frame of the trolley, with the cam having a serrated braking edge that will engage the cable or rail when biased by the weight of the falling worker. The fall arrest systems of these trolleys are unidirectional in that they are intended to be activated only in a single direction and pivoting the cam in the opposite direction will not result in any braking action—on the contrary, pivoting the cam in the opposite direction is usually accomplished to disable the cam's braking action to allow the worker to move along the cable or rail. The unidirectional quality of fall arrest systems of trolleys usable along vertical cables is to be expected since the fall may of course only occur in a single direction on vertical cables, namely along the cable.
Prior art trolleys for use along inclined cables or rails are used by workers that work on inclined surfaces such as roofs. These trolleys also have a common feature: many include a pair of cams that will allow the braking action to occur when the user falls along the inclined surface being worked on. That is to say, these trolleys are also activated upon the worker falling parallel to the lifeline. Some of these trolleys are bidirectional in that they are intended to be activated in either one of two directions, both of them parallel to the lifeline. This is why they are provided with two cams instead of a single one. This way, if a worker works on both opposite sides of an inclined roof, he may cross over from one side that is inclined in a first direction to the other that is inclined in the other direction along a single lifeline, and his fall arrest system will remain operable at any point of his travel.
No prior art trolley however is intended for use on an inclined lifeline such that the fall arrest system will be activated when a user falls transversally away from the lifeline. This may occur for example if the user works along a structure where an inclined lifeline is installed, i.e. where the cable supports are vertically offset, however without having a correspondingly inclined structure underfoot. For example, if the lifeline extends along a space frame truss that is itself inclined, then the user may fall vertically downwards at any point along the space frame truss between or besides the truss members. This is different from workers working on roofs where a vertical fall won't occur: a worker may accidentally slide along the roof and it is this sliding that must be arrested, parallel to the roof and parallel to the lifeline. However the worker will not fall “through” the roof.
Another feature of many prior art trolleys, both the ones for use along vertical lifelines and the ones for use along horizontal lifelines, is a biasing member, usually a spring, that continuously forces the braking device into a default braking position. This prevents the trolley from moving along the lifeline when the user releases it. To move the trolley along the lifeline, the worker must enable this movement by disabling the brake. This is accomplished by manually pulling on the cam or other brake member to force it away from its braking position against the bias of the spring. However, an important problem can, and sometimes does, result when a fall occurs while the worker is disabling the braking device to move: in his fall, the worker does not think to release the braking device, and the trolley, free to move along the lifeline, follows the worker in his fall. It should be noted that when falling, many workers will have a reflex to contract their muscles under the sudden stress of the event, and consequently not releasing the braking device held in a disabled position is far from improbable.